Liquid crystal displays are used for personal computers, TVs, monitors, cellular phones, PDAs, and so on. Dyed polyvinyl alcohol-based films have been used for polarizers of liquid crystal displays and so on, because they have a high level of both transmittance and degree of polarization. For example, such polarizers are produced by a process that includes subjecting a polyvinyl alcohol-based film to each of swelling, dyeing, crosslinking, and stretching processes in a bath, then subjecting the film to a washing process, and then drying the film. Such polarizers are generally used in the form of polarizing plates, which are each composed of a polarizer and a protective film, such as a triacetylcellulose film, bonded to one or both sides of the polarizer with an adhesive.
In recent years, liquid crystal displays have become more sophisticated, and liquid crystal panels are required to have higher contrast such that high visibility can be achieved. Specifically, blacker in black viewing and whiter and brighter in white viewing are desired, and, therefore, further improvement in the polarization performance of polarizers is required. Accordingly, satisfying both high degree of polarization and high transmittance has become very important for polarization performance.
In order to obtain such polarizers, a large number of methods have been proposed. For example, it is proposed that a polarizer manufacturing method should include allowing an unoriented, polyvinyl alcohol-based film to swell in a swelling bath, then adsorbing iodine to the film in an iodine dyeing bath, and further subjecting the film to treatments such as crosslinking and stretching in an aqueous solution containing boric acid (Patent Document 1). It is also proposed that a polyvinyl alcohol-based film should be allowed to swell and then subjected to a process including heating, humidifying, then performing an iodine dyeing treatment, further performing weak stretching, and then strong stretching (Reference Document 2).